The present invention relates to a novel magneto-optical recording medium capable of giving a large Kerr rotation angle and a high C/N (carrier-to-noise) ratio by virtue of the high transparency and high refractive index of the dielectric layers to give a very high recording density and stability of the magnetic properties.
Along with the progress of the social activity led by a flood of information, magneto-optical memory devices suitable for erasing and rewriting of information records using a laser beam are high-lighted in recent years. A magneto-optical recording medium generally has a laminar structure, formed in turn on a transparent substrate plate, composed of a first dielectric layer, a magnetic recording layer, a second dielectric layer and a metallic reflecting layer. Conventionally, the magnetic recording layer is formed from an amorphous alloy of a rare earth metal, e.g., terbium, and a transition metal, e.g., iron and cobalt. These conventional rare earth-based amorphous alloys, however, are not quite satisfactory because the Kerr rotation angle thereof is generally not large enough so that the C/N ratio of recording cannot be sufficiently high.
Accordingly, it is a usual practice that a layer of a dielectric material such as silicon monoxide SiO, silicon nitride SiN, aluminum nitride AlN and the like is formed on the surface of the amorphous magnetic recording layer and the thickness of the dielectric layer is controlled to be .lambda./4 n, in which .lambda. is the wavelength of the laser beam and n is the refractive index of the dielectric material, so as to apparently increase the effective Kerr rotation angle. The improvement obtained by such a means relying on the so-called enhancement effect is still insufficient even by sandwiching the magnetic recording layer with two dielectric layers so that it is eagerly desired to develop a magneto-optical recording medium having a markedly enhanced effective Kerr rotation angle. In this regard, it is an essential requirement that the dielectric material has a refractive index and transparency as high as possible.
Another important problem in the conventional magneto-optical recording media is the relatively low stability of the magnetic properties of the recording layer thereof in the lapse of time, especially, at a high temperature and in a highly humid atmosphere. This disadvantage is presumably due to the high susceptibility of the rare earth metal as a constituent of the magnetic alloy to the attack of the atmospheric oxygen. In this regard, the dielectric layers formed on the magnetic recording layer should desirably serve as a protecting layer against air oxidation while conventional dielectric materials used in the dielectric layers are not always quite effective as a barrier against air oxidation. For example, a layer formed from silicon oxide SiO acts on the rare earth metal rather as an oxidizing agent to oxidize the metal. Other conventional dielectric materials such as silicon nitride and aluminum nitride are of course free from such a problem of oxidation against stability of the magnetic properties but they have another problem of poor mechanical properties to be liable to the formation of cracks in the course of deposition of the layer.
A proposal has been made by M. Asano, et al. in IEEE Trans. Magn., MAG-23, page 2620 (1987) for the use of boron nitride BN as a material of the dielectric layers of a magneto-optical recording medium. This material is indeed excellent in the performance as a dielectric material along with a high refractive index and transparency. Disadvantageously, however, the dielectric layer formed from boron nitride cannot be perfectly amorphous even when it is formed by the method of sputtering and has problems in respects of the homogeneity of the composition and smoothness of the surface along with the relatively low durability.